Can anyone give me a reason why a petrol (Honda) mower i've just been given thats about ten years old doesn't pick up grass into the grass box.

The mower cuts great but leaves all the grass behind giving me double handling. I've looked at the blade and it seems ok though a little worn so i've sharpened it. Theres there's nothing in the cavity leading to the grass box, so I'm stumped. Any ideas will be greatly appreciated